The electric vehicle has been developed rapidly. The electric vehicle employs a power battery as the power source thereof and is driven by an electric motor, thus avoiding limitation of the engine and reducing the environmental pollution.
Conventionally, the electric vehicle may be driven by a driving motor and a reducer with a constant transmission ratio. It is well known that the output torque of the driving motor is large and the efficiency is low at a low speed. With the increasing of the speed, the efficiency of the driving motor is increased gradually, but the output torque is decreased.
With the electric vehicle comprising the reducer with a constant transmission ratio, if the reducer is designed to have a large output torque, the efficiency is increased slowly with the increasing of the speed of the driving motor, and consequently the power consumption of the vehicle is high at a high speed. If the reducer is designed to have a high speed, the efficiency is increased fast with the increasing of the speed of the driving motor. However, since the transmission ratio of the reducer is low, the torque transmitted to the wheels is too low to satisfy the requirement of starting or climbing of the vehicle. As a result, the acceleration ability of the vehicle is poor and the power consumption is high at a low speed of the vehicle. For example, the electric bus may need to frequently stop, start or accelerate, which may cause the efficiency of the driving motor low. Therefore, the driving system of the conventional electric vehicle is difficult to meet the work conditions at both low and high speeds simultaneously, so that the efficiency of the driving motor is low, which may decrease the cruising range of the electric vehicle. As a result, the conventional electric vehicle may not be suitable for the complicated road conditions.